1. Field of the Invention
This invention relates to a position feedback system adapted to determine the working position of a machine tool or the like in a closed loop by means of a numerically controlled apparatus, and more particularly to a position feedback system adapted to eliminate the failure of operation of a numerically controlled apparatus due to the mechanical vibration of the machine tool side or the like.
2. Description of the Prior Art
Recently, the control of a machine tool or the like utilizing a numerically controlled apparatus (hereinafter referred to as "NC apparatus") has been carried out by a closed-loop control system which is adapted to accomplish the automation of working and the working with a high precision by comparing a current working position data on the machine tool side returned to the NC apparatus with a working position data stored in the NC apparatus and feeding the deviation therebetween as a working command signal to the machine tool side.
An example of such a closed-loop control system used for this purpose is shown in FIG. 1 which is adapted to detect the working position of a machine tool side by means of a linear-type measuring device and return the position data in the form of a working command signal to the NC apparatus side.
The system shown in FIG. 1 includes a working table 1 of a machine tool on which a work 2 is securely disposed and a scale 4 of a measuring device 3 is fixed.
The measuring device 3 includes an encoder 5 for detecting the movement of the scale 4, which is fixed at the fixed bed 6 of the machine tool.
Reference numeral 7 designates an NC apparatus which is adapted to generate a working command signal, which allows a driving source 8, such as, for example, a motor to be driven to move the table 1. With the movement of the working table 1, working of the work 2 is proceeded by a working shaft 9, and the working position is read by the measuring device 3 and is returned through a feedback unit 10 to the NC apparatus side 7.
In other words, the relative movement between the scale 4 and the encoder 5 by the command of the NC apparatus 7 is returned as a current positional signal through the feedback unit 10 to the NC apparatus side 7 so that desired working may be automatically accomplished by the NC apparatus 7.
In this instance, it is required that the measuring device 3 comprising the scale 4 and encoder 5 discriminates and feeds the moving direction of the scale and encoder as well as the relative movement between the both to the side of feedback unit 10.
For example, every time when the scale 4 is relatively moved by a unit length (1 .mu.m-several tens .mu.m) with respect to the encoder 5, the encoder 5 is caused to generate two signals of which phases are different by a degree of 90.degree. from each other. This is carried out, for example, by arranging movement detecting devices at positions in the encoder 5 different in phase by 90.degree. from each other. Further, the two signals thus generated different in phase by 90.degree. (hereinafter referred to as "A phase signal" and "B phase signal", respectively) are treated to carry out discrimination of the direction and detection of the movement.
More particularly, when the A phase signal and B phase signal respectively shown in FIGS. 2(a) and 2(b) are generated in the encoder 5 with the movement of the scale 4, the A phase signal is supplied to a differentiating circuit 51 shown in FIG. 3 and the leading portion thereof is differentiated. The B phase signal is introduced to a phase inverting circuit 52. When the A phase signal rises while the B phase signal is during period of time "1", a gate circuit 53 is opened so that a differentiation pulse in synchronism with the rising of the A phase signal is supplied to the terminal t.sub.1 as shown in FIG. 2(c). When the A phase signal rises while the B phase signal is during a period of time "0", the B phase signal having an inverted phasea causes a gate circuit 54 to be opened, which results in a differentiation pulse in synchronism with the rising of the A phase signal being supplied to the terminal t.sub.2 as shown in FIG. 2(d). The A phase signal and B phase signal have correlation that the A phase signal is generated with a phase lag of 90.degree. with respect to the B phase signal, when the working table 1 is moved in the left direction with respect to the fixed bed 6 as shown in FIG. 1, whereas it is generated with a phase lead of 90.degree. with respect to the B phase signal when the working table 1 is moved in the right direction.
Thus, supposing that the movement of the working table 1 in the left direction is positive and that in the right direction is negative, the direction and amount of movement of the table 1 can be measured by up-counting of a counter by means of a pulse generated at the terminal t.sub.1 and down-counting of the counter by means of that generated at the terminal t.sub.2.
Further, an increase in resolution of the measurement is carried out by electrically equally dividing the A phase signal and B phase signal to form a plurality of pulses for up-counting or down-counting during one cycle of each phase signal.
The positional data measured by the measuring device 5 is treated in the feedback unit 10 and then returned to the NC apparatus side 1, to thereby form a closed loop.
However, in the control system described above, the NC apparatus is restricted in the capacity of reading pulse signals. Thus, when pulse signals are subsequently supplied at highly short intervals from the feedback unit 10 to the NC apparatus 7, the NC apparatus fails to read the signals to cause the operation of the whole system to be stopped.
More particularly, in FIG. 1, when a tool is mounted on the working shaft 9 and deep cuts are carried out on the work 2, vibration due to cutting is transmitted through the working table 1 directly to the scale 4. The encoder 5 detects the vibration in the form of displacement of the scale 4 and feeds it in the form of a counting pulse through the feedback unit 10 to the NC apparatus 7.
The vibration generally has a highly short cycle, therefore, the counting pulse supplied to the NC apparatus 7 has a highly narrow width and a short cycle period. Further, the direction of the movement is also subjected to such vibration so that the NC apparatus 7 may not follow the supply of the counting pulse to fail to read it.
More specifically, this causes the advance and delay of the A and B phase signals to be generated at random as shown in FIGS. 4(a) and (b), resulting in a count-up signal shown in FIG. 4(c) and a count-down signal shown in FIG. 4(d) being repeatedly supplied to the NC apparatus. Also, the counting pulse has a highly short cycle. Thus, the NC apparatus fails to follow the supply of the pulses and read the pulses.